Polyester production from terephthalic acid (TPA) or its esters, such as dimethyl terephthalate (DMT), and glycols is known. This has been accomplished by stage-wise melt polymerization of the dihydroxy ester of the bifunctional carboxylic acid, or low molecular weight oligomers thereof, under successively higher vacuum conditions. In order for the polymerization to continue to the degree needed for most commercial applications, the condensation by-products, especially the glycols, must be removed from the reaction system at vacuums as high as 1-3 mm Hg. Such processes require costly high vacuum equipment, multistage steam jets to create the vacuum, and N.sub.2 purged seals and flanges to minimize leakage of air into the system. Condensate from the steam jets and organic by-products from the system end up as a waste water stream that requires treatment and contributes to volatile organic emissions to the air. The present invention provides a less costly polymerization process that can be carried out at atmospheric pressure and in a closed loop configuration that eliminates volatile organic emissions and the waste water discharge.
Atmospheric pressure processes to conduct polymerization, without employing vacuum and using an inert gas, have been disclosed in the prior art, but these have several drawbacks.
U.S. Pat. No. 2,973,341 (Hippe) discloses a continuous process for the production of polyester condensate and an improved continuous process for making polyethylene terephthalate from dimethyl terephthalate and ethylene glycol. The process employs liquid dimethyl terephthalate and mixes with it ethylene glycol, in an excess molar ratio of 1.5:1, to form a liquid reaction mixture in a first stage below the transesterification temperature and then carrying the liquid reaction mixture through three separate temperature controlled stages. Transesterification occurs in the second stage at a temperature of not more than 197.degree. C.; vaporous reaction products are removed in the third stage at 197.degree. C. to 230.degree. C. by passing an inert gas through the liquid reaction mixture; polycondensation occurs in the fourth stage at 230.degree. C. to 255.degree. C. for a period of time sufficient to produce a filament forming polyethylene terephthalate condensate while again passing an inert gas through the liquid reaction mixture. Ethylene glycol by-product can be recovered from the fourth stage and recycled to the second stage of the reaction.
U.S. Pat. No. 3,545,520 (Siclari et al.) discloses an apparatus for stripping substances and lightweight fractions from polymers including a means for introducing an inert gas counter current to the polymeric material with the consequent increase in viscosity of the polymers. The apparatus permits recycling a portion of the material removed from the vessel so that the material can be recycled into the reaction container.
U.S. Pat. No. 3,469,618 (Siclari et al.) discloses a method for stripping off volatile fractions from polyamides and polyesters involving feeding material in the form of droplets or liquid threads though an inert gaseous atmosphere, while recirculating that atmosphere.
U.S. Pat. No. 3,110,547 (Emmert) discloses a process for preparing a linear condensation polyester. In one embodiment of the invention, the polymer is extruded downwardly through a chamber while passing a current of inert gas, such as nitrogen, through the reaction vessel at a rate sufficient to keep the glycol partial pressure below 2 mm Hg while maintaining a temperature between 300.degree. C. and 400.degree. C. in order to rapidly finish the polymer by converting the polymer having a degree of polymerization of from about 15.degree. to 35.degree. to a finished polymer with a degree of polymerization of about 70.degree..
U.S. Pat. No. 3,390,135 (Seiner) discloses a continuous polyesterification process by direct esterification of dicarboxylic acids and polyhydric alcohols, and contacting the reaction product with a nonreactive gas to remove the water of esterification.
U.S. Pat. No. 3,480,587 (Porter) discloses a polyester preparation process in which a lower molecular weight prepolymer is polymerized by conducting polycondensation in narrow tubes under conditions of turbannular flow achieved with an inert gas medium flowing cocurrently at high velocities.
French A, 239,649 (Bayer) discloses a continuous process for preparing polybutylene terephthalate wherein monoesters or low viscosity polybutylene terephthalate is polymerized by cocurrently transporting it with a heated inert gas in the form of two phase annular flow through a long, narrow, helical tube of 3 to 100 nm (0.1 to 3.9 inch) diameter in which the inert gas flows in a velocity of 20-300 m/s (equal to 66 to 984 ft/second).
European Patent A, 0,182,351 (Mitsubishi) discloses a polyester process in which the ester or its oligomer is polymerized in the form of fine, 0.015 to 0.5 mm particles sprayed into an inert gas stream.
U.S. Pat. No. 5,064,935 (Jackson et al.) discloses a continuous process for preparing polybutylene terephthalate oligomer or prepolymer for feeding into a conventional polycondensation for PBT polymer. The prepolymer is prepared by feeding the reaction mass from a prior transesterification step into the top of a countercurrent column reactor through which a heated inert gas is passed upward by introducing it at the bottom.
The processes disclosed above, however, suffer from one or more drawbacks such as (1) only a low molecular weight oligomer or a prepolymer is produced; (2) the quantity of inert gas used is very large to be economical; (3) the reactor size might be too large to be feasible for commercial scale operation; (4) the inert gas employed is not recycled in a closed loop to eliminate emissions; (5) a prepolymer of sufficiently high molecular weight is required to achieve high molecular weight polyester required for commercial application; (6) inert gas velocities employed are too high to be feasible for commercial scale production or a high pressure is required. Because of such drawbacks, the processes presently practiced for commercial production of polyester continue to be conducted under high vacuum as described above.
The object of the present invention is to provide an improved atmospheric pressure process for continuous or batchwise production of polyesters, especially poly(alkylene terephthalates), particularly poly(propylene terephthalate), and poly(butylene terephthalate) of high molecular weight.